Elastography is conventionally known that measures the hardness of living tissue and visualizes the distribution of the measured hardness. Elastography is utilized for diagnosing diseases in which the hardness of living tissue changes in accordance with the degree of progress of lesions such as hepatocirrhosis. In elastography, methods for evaluating hardness by displacing living tissue are broadly divided into the following two.
The first method is a method that visualizes relative hardness from the magnitude of distortion at points within a scanning section that is observed when living tissue is pressed and released from the surface of the body with an ultrasonic probe. The second method is a method that gives acoustic radiation force or mechanical vibrations to living tissue from the surface of the body, generates displacements by a shear wave, and observes displacements at points within a scanning section over time, thereby determining the propagation speed of the shear wave and determining a modulus of elasticity. In the first method, the local magnitude of distortion depends on the magnitude of manual movement of the ultrasonic probe, and it is evaluated whether a region of interest is softer or harder than the surroundings. In contrast, the second method can determine the absolute modulus of elasticity of a region of interest.
When a displacement is generated in living tissue by giving acoustic radiation force in the second method, however, depending on the beam shape of a burst wave (push pulse) for displacement generation, a displacement distribution suitable for the observation of the shear wave cannot be given especially in a shallow part. Specifically, living tissue over a wide area displaces by the push pulse, and displacement propagation in a lateral direction cannot be captured with high precision. This causes artifacts in a hardness image indicating the hardness of the living tissue.